The RCC is an inverter repeating the operation in which a stored energy in a coil is released to generate voltage-oscillating energy, and the generated voltage-oscillating energy is rectified to supply it to the load.
A switching power supply of the RCC type stores excited energy inside a transformer during a turn-on period of a main switching device and charges a capacitor by a current obtained from a voltage induced in a control winding of the transformer and a feedback current supplied from the secondary side. Then, when a charged voltage reaches a predetermined voltage, a control switching device turns off a control terminal of the main switching device. During the turn-off period, the excited energy stored inside the transformer is outputted to the secondary side. After all the excited energy is outputted, a ringing pulse which generates in a control winding of the transformer is fed back to the control terminal of the main switching device to turn on the main switching device again, thereby carrying out steady oscillation.
The heavier the load is, the turn-off period and the turn-on period become longer automatically. That is, since a switching frequency drops, an output voltage on the secondary side can be maintained to a predetermined constant voltage. This eliminates the need for such an intricate control circuit as that in a switching power supply of the pulse width modulation (PWM) type and the need for a power supply circuit for operating the control circuit and generating a voltage that will be a reference voltage for a pulse width. Therefore, the power supply of the RCC type, which is preferable in terms of cost reduction, has been widely used.
Note that, as prior art related to a switching power supply of the RCC type, there exists Japanese Laid-Open Patent Application No. 2000-333448 (Tokukai 2000-333448; published on Nov. 30, 2000) and Japanese Laid-Open Patent Application No. 46480/1999 (Tokukaihei 11-46480; published on Feb. 16, 1999).
FIG. 8 is an electrical diagram of a typical and conventional switching power supply 1 of the RCC type. This switching power supply 1 primarily has an arrangement in which a main switching device q is connected in series to a primary winding n1 of a transformer t so that the output of a control winding n3 of the transformer t is fed back to the main switching device q via a control circuit 2 for continuous oscillations.
A direct current obtained by rectifying a commercial alternating current by a power supply circuit (not shown) or a direct current from a battery is supplied between input terminals p1 and p2, and a DC supply voltage is outputted between a power supply line 3 on the high level side and a power supply line 4 on the low level side.
Between the power supply lines 3 and 4, a series circuit with the primary winding n1 of the transformer t and the main switching device q is connected as described above. The switching device q is realized by, for example, a bipolar transistor and a field effect transistor. In this example in FIG. 8, a field effect transistor is adopted. Also, the control circuit 2 is connected between the power supply lines 3 and 4 via a starting circuit 5.
The starting circuit 5 is composed of two stages of starting resistors r1 and r2 so that even when a short circuit occurs at a short/open test for the circuit components, a high voltage for use in the test would not be directly applied to the control circuit 2.
When power is applied, i.e. a DC supply voltage is applied to between the input terminals p1 and p2, a junction capacitor inside the main switching device q and a DC-blocking capacitor c1 starts being charged. Note that, a charging rate in this case is determined in accordance with divided voltage values of the starting resistors r1 and r2 and a starting resistor r3 inside the control circuit 2, and capacitances of the junction capacitor inside the main switching device q and the DC-blocking capacitor c1. This causes a potential at the gate of the main switching device q to start increasing. When the potential at the gate of the main switching device q reaches an ON threshold voltage, e.g. 3V or higher, the main switching device q turns on. This causes an upward voltage in FIG. 8 to be applied to the primary winding n1, storing excited energy.
In the control winding n3 of the transformer t, an upward voltage in FIG. 8 is induced when the main switching device q turns on. Further, the induced voltage causes a current to be supplied via a bias resistor r4 and the DC-blocking capacitor c1 to the gate of the main switching device q. This maintains a turn-on state of the main switching device q.
Moreover, a current obtained from the upward voltage which has been induced in the control winding n3 when the main switching device q turns on is supplied via a phototransistor tr1 of a photocoupler pc in the control circuit 2 to one terminal of a capacitor c2. The other terminal of the capacitor c2 is connected to the power supply line 4 on the low level side. Therefore, the capacitor c2 is charged by the upward voltage, and the higher an output voltage on the secondary side becomes, the higher a charging current through the phototransistor tr1 becomes, and the more quickly the voltage between the terminals of the capacitor c2 increases.
The voltage between the terminals of the capacitor c2 is supplied to the base of a control transistor tr2 between the gate and source of the main switching device q. When the voltage between the terminals of the capacitor c2 reaches an ON threshold voltage, e.g. 0.6V or higher, the control transistor tr2 turns on. This causes the potential at the gate of the main switching device q to drop sharply, resulting in turn-off of the main switching device q.
Therefore, the higher the output voltage on the secondary side becomes, i.e. the lighter the load is, the more quickly the voltage between the terminals of the capacitor c2 increases, and the more quickly the main switching device q turns off. To the capacitor c2, the current that has been induced in the control wiring n3 is supplied via a resistor r5. A series circuit with the resistor r5 and the capacitor c2 is connected in parallel to the control winding n3, which makes up an overcurrent protection circuit. Even when the short circuit occurs on the secondary side, the overcurrent protection circuit limits a turn-on period of the main switching device q to a predetermined length, allowing for the protection of the main switching device q.
When the main switching device q turns off, a downward voltage in FIG. 8 is induced in the control winding n3. This induced voltage causes a current flow in the series circuit with the capacitor c2 and the resistor r5, decreasing the charges stored in the capacitor c2 in preparation for a next turn-on operation of the main switching device q.
Meanwhile, right after the main switching device q turns off, the excited energy that has been stored in the transformer t starts to be outputted to a secondary winding n2, and a direct current is induced in the secondary winding n2. Then, the direct current induced in the secondary winding n2 is supplied via a diode d1 to a smoothing capacitor c3, is smoothed by the smoothing capacitor c3, and is outputted via the output power supply lines 6 and 7 from output terminals p3 and p4 to a load circuit (not shown).
Further, between the output power supply lines 6 and 7, there is a voltage detector circuit 8. The voltage detector circuit 8 is composed of a voltage-dividing resistor, a photocoupler (not shown), and other components, and a light-emitting diode of the photocoupler turns on a light with the luminance corresponding to the output voltage on the secondary side. Then, owing to this lighting, a value of the outputted voltage on the secondary side is fed back via the phototransistor tr1 to the primary control circuit 2 on the primary side.
Thus, when all the excited energy stored in the transformer t is released from the secondary winding n2, the energy stored in a parasitic capacitor c4 which is included in the primary winding n1 is released from the primary winding n1, resulting in the occurrence of electrical resonance (ringing) between the parasitic capacitance c4 and the primary winding n1.
A ringing pulse caused by the ringing is transferred to the control winding n3 that is magnetically coupled to the primary winding n1, and is supplied via the bias resistor r4 and the DC-blocking capacitor c1 to the gate of the main switching device q. The ringing pulse supplied to the gate of the main switching device q is set so as to be the ON threshold voltage or higher of the main switching device q under the steady oscillation state. This causes the main switching device q to turn on. In such a manner, the main switching device q repeats ON/OFF operation, and the switching power supply 1 goes from the initial oscillation state to the steady oscillation state.
In the above-arranged switching power supply 1 of the RCC type, the initial oscillation state of depending on the current supplied from the starting resistors r1 and r2 exits for a predetermined time period at the start of the switching power supply 1. Thereafter, the switching power supply 1 goes from the initial oscillation state to the steady oscillation state on its own.
However, depending on the DC supply voltage and the state of load at the start, there might occur a poor starting that the switching power supply 1 is stabilized without going from the initial oscillation state to the steady oscillation state.
Especially, when the switching power supply 1 starts with a low DC supply voltage or a heavy load, the output voltage on the secondary side does not reach a targeted value. Therefore, a peak value of the ringing pulse becomes below the ON threshold voltage (the ringing pulse does not reach the ON threshold voltage), and the main switching device q does not turn on until the voltage obtained in the starting resistors r1 and r2 becomes the ON threshold voltage or higher again, and the switching power supply 1 becomes stable in this state, causing a poor starting. Details of this will be described with reference to FIGS. 9 through 11.
FIG. 9 is a view showing waveforms for the circuit components under the initial oscillation state, and FIG. 10 is a schematic view of the waveforms in FIG. 9. Vds represents a drain voltage waveform of the main switching device q where the source is connected to GND potential. Is represents a waveform of a current released from the secondary winding n2. Vgs represents a gate voltage waveform of the main switching device q.
FIG. 11 is an enlarged view of the gate voltage waveform for the main switching device q under the initial oscillation state. At the end of a period in which a current is released from the secondary winding n2, which is indicated by the reference mark w1, the ringing pulse which occurs between the parasitic capacitor c4 and the primary winding n1 appears in the control winding n3 that is magnetically coupled to the primary winding n1. The ringing pulse corresponds to the peak indicated by the reference mark a.
The period indicated by the reference mark w2 is a period in which the junction capacitor inside the main switching device q and the capacitor c1 are charged by the current supplied from the starting resistors r1 and r2, and the potential at the gate of the main switching device q increases moderately. A peak in the period indicated by the reference mark w3 is a state in which owing to the potential at the gate of the main switching device q being ON threshold voltage or higher, the main switching device q turns on, thereby causing a current flow in the primary winding n1, resulting in the upward voltage in FIG. 8 induced in the control winding n3. The voltage induced in the control winding n3 is supplied to the gate of the main switching device q via the bias resistor r4 and the DC-blocking capacitor c1. This maintains the turn-on state of the main switching device q.
The peak value of the ringing pulse indicated by the reference mark a is lower than the ON threshold voltage of the main switching device q indicated by the reference mark Vth because the output voltage on the secondary side does not reach a targeted value in the early stage of the power start-up. Therefore, the ringing pulse does not cause the turn-on of the main switching device q. Here, the period indicated by the reference mark w2 is needed to charge the junction capacitor inside the main switching device q and the capacitor c1 by the current supplied from the starting resistors r1 and r2 to further increase the potential at the gate of the main switching device q and to make the peak value of the ringing pulse indicated by the reference mark a reach the ON threshold voltage or higher.
After the lapse of the period indicated by the reference mark w2, the peak value of the ringing pulse gradually increases with the increase of the output voltage on the secondary side and finally becomes the ON threshold voltage Vth or higher. At this point, the main switching device q turns on only with the ringing pulse, without the help of the current from the starting resistors r1 and r2, and the switching power supply 1 goes into the steady oscillation state. However, in the case of the aforementioned low DC supply voltage and heavy load, the output voltage on the secondary side does not reach a targeted value, and the peak value of the ringing pulse indicated by the reference mark a becomes stable without reaching the ON threshold voltage Vth, resulting in a poor starting of the switching power supply 1.
In order to improve such a poor starting, there is a technique of setting low resistance of the starting resistors r1 and r2 to increase the current supplied from the starting resistors r1 and r2. This shortens a stand-by period before the potential at the gate of the main switching device q reaches the ON threshold voltage Vth (i.e. the period indicated by the reference mark w2) and relatively increases the number of times ON/OFF operation is performed per unit of time, thereby facilitating the output voltage on the secondary side reaching a targeted value.
However, a low resistance of the starting resistors r1 and r2 results in the problem of increasing power consumed in the starting resistors r1 and r2 and of significantly decreasing conversion efficiency especially under light load.